As exemplified in U.S. Pat. No. 5,779,417 (ITW Case 8048) a fastener of the type noted above is used for fastening a sheet-metal plate, which may be conveniently called a faying plate, to a substrate, which usually is a metal substrate. If the substrate is another sheet-metal plate, the substrate plate may be conveniently called a tapping plate. Commonly, the faying plate is prepared, as by punching, with a hole which may be conveniently called a clearance hole and the substrate is prepared, as by punching or drilling, with a hole which may be conveniently called a tapping hole, and which is smaller in cross-section than the clearance hole. Commonly, the shank is adapted to pass freely through the clearance hole, to enter the tapping hole at a tapered tip, and to tap a complementary thread around the tapping hole, via a self-tapping thread on the shank, when the fastener is driven.
Among other data characterizing a fastener of the type noted above, it is convenient to refer to a tapping torque, which is the torque required for the self-tapping thread to tap the complementary thread when the fastener is driven rotatably. Moreover, it is convenient to refer to a stripping torque, which is the torque required for the self-tapping thread to strip the complementary thread so that the shank rotates freely within the tapping hole.
Commonly, fasteners of the type noted above are produced in large quantities, from which statistically meaningful samples are drawn for testing. Among other data obtainable from testing of any given sample, it is convenient to refer to a statistical maximum tapping torque, which is the maximum torque required for the self-tapping thread of any of the tested fasteners of the sample to tap such a complementary thread. Moreover, it is convenient to refer to a statistical minimum stripping torque, which is the torque required for the self-tapping thread of any of the tested fasteners of the sample to strip the complementary thread so that the shank rotates freely within the tapping hole.
In many applications, such fasteners are driven via pneumatically or electrically powered driving tools, each of which is arranged to apply a driving torque to a fastener head. Desirably, each such tool is adjusted so as to stall or so as to stop driving when a nominal maximum driving torque is applied, which is higher than the statistical maximum tapping torque for such fasteners and lower than the statistical minimum stripping torque for such fasteners. Since such tools tend to be imprecisely adjustable, it is desirable for the statistical minimum stripping torque to be substantially higher than the statistical maximum tapping torque for any given quantity of threaded fasteners of the type noted above.